1. Field of the Invention
The present invention relates generally to a method of delignifying and bleaching cellulosic pulps. More particularly, the invention relates to a totally molecular chlorine-free method of delignifying and bleaching chemical cellulosic pulps using a combination of peracetic acid, ozone and oxygen.
2. Description of the Prior Art
Conventional methods of processing cellulosic pulps generally include multiple delignification steps. In order to obtain a final pulp of sufficient brightness, most industrial processes rely upon bleaching the pulp with chlorine-based bleaching agents. Molecular chlorine and, more recently, chlorine dioxide have been used in these processes. However, the use of chlorine-based bleaching agents has met with increasing objections and strict legislation has been proposed in an effort to force replacement of such chlorine bleaches in lignocellulose processes with non-chlorine based bleaches.
The pulp and paper industry has devoted substantial efforts to the development of chlorine-free or reduced chlorine bleaching processes. One such effort has been the development and implementation of oxygen bleaching systems. U.S. Pat. No. 3,832,276 to Roymoulik discloses one oxygen delignification process. Oxygen itself, however, cannot produce pulps of sufficiently high brightness and quality for many commercial applications. U.S. Pat. No. 4,626,319 to Kruger et al. teaches the use of oxygen and hydrogen peroxide in the dilignification and bleaching of cellulose. The ineffectiveness of oxygen in the bleaching of pulps has led other researchers to investigate the use of ozone as a bleaching agent, either alone or following an oxygen bleaching stage. Oxygen and ozone bleaching of cellulosic pulps are generally carried out separately because ozone is intolerant of alkaline conditions and oxygen is inefficient in an acidic environment.
For a review of the literature relating to the ozone bleaching of fibrous materials reference can be made to Medwich v. Byrd, Jr. "Delignification and Bleaching of Chemical Pulps with Ozone: a Literature Review" Tappi Journal, 207-213, March 1992.
U.S. Pat. No. 4,080,249 to Kempf teaches the use of a mixture of about 0.1 to 20% ozone in oxygen or air to delignify and bleach cellulose pulp. The delignification and bleaching sequence may include a first stage where oxygen is employed as a reactant in the presence of an alkali.
U.S. Pat. No. 5,074,960 to Nimz et al. teaches the use of ozone in the presence of a C.sub.1-3 fatty acid to remove lignin from a cellulosic or lignocellulosic pulp. The ozone is present at a concentration of about 0.1 to 10% in a gaseous phase consisting of air or oxygen.
In the prior bleaching processes which employ oxygen and ozone in separate stages or individual unit operations, there is a substantial change in the process pH from strongly alkaline for oxygen delignification to very acidic for ozone delignification. The control of pH is critical at these two extremes. For this reason, the oxygen and ozone treatments are separated. They are conducted in separate towers and the pulp is washed between each treatment. In ozone treatments, oxygen is presented as a carrier-gas and occupies almost 90% of the total weight of the gas stream but oxygen is essentially inert and inactive under the process conditions employed in these reactions.